Theoretical thermal efficiency of a stirling engine is determined by the temperature of a high temperature section and of a low temperature section, and the higher the temperature of the high temperature section and the lower the temperature of the low temperature section, the higher the thermal efficiency is. The stirling engine is a closed cycle engine, and heats/cools working gas from the outside, thus heating and cooling of the working gas need to be performed through a wall surface of the high temperature section and of the low temperature section, and further a material of high heat conductivity is required in order to increase heat exchange rate of the high temperature section and of the low temperature section. As the working gas, helium gas or hydrogen gas is normally used. Since the working gas circulates at high pressure, a flow path for the working gas is required to have heat resistance property, pressure tightness, oxidation resistance, corrosion resistance, high creep strength, and high heat fatigue strength. For this reason, as a heater tube configuring a cylinder and high-temperature side heat exchanger, there has been conventionally used heat-resistant alloy steel such as HR30 (Japanese Industrial Standards), SUS310S (Japanese Industrial Standards), Inconel (trademark), Hastelloy (trademark), and the like having excellent corrosion resistance and heat resistance properties, but there is a problem that these alloy steels are extremely expensive. Moreover, in such a case, the members configuring the high temperature section, and the members subjected to high temperatures by receiving heat from the high temperature section are subjected to limitations in heating temperatures, depending on metallic materials. For example, under a high-pressure condition in which the pressure of operation gas reaches 3 MPa, it is considered that the limit of the heating temperature is approximately 700° C. from the perspective of durability, due to the occurrence of a creep of abovementioned metallic materials, hence it is difficult to achieve high efficiency if the heating temperature is increased higher than the limit.
Further, in a conventional stirling engine, it is necessary to create the high temperature section by weldbonding a number of heat-resistant alloy tubes, through which working gas passes, to an expansion space head portion by means of brazing so as to allow the heat-resistant alloy tube to protrude, in order to obtain more heat transmission areas. However, leakage of the working gas may occur due to a seal failure, and, since a number of heat-resistance alloy tubes are required, the structure becomes complicated and the cost becomes high.
On the other hand, in the member for connecting the high temperature section and the low temperature section in the stirling engine, an end of the high temperature section is required to maintain high temperature and an end of the low temperature section is required to maintain low temperature to keep a large temperature difference therebetween, and the high temperature of the high temperature section and the low temperature of the low temperature section are close to each other, thus it is desired that members having high adiathermanous and low heat conductivity be used to configure the stirling engine. However, in the conventional Stirling engine the member for connecting the high temperature section and the low temperature section is integrally configured with a high temperature section composed of high-nickel alloy or a stainless material having excellent heat resistance property and heat conductivity, thus there is a problem that a large heat loss occurs due to conduction of heat through a member wall connecting the high temperature section and the low temperature section.
As described above, the material configuring the high temperature section is required to have excellent heat resistance property, and also required are contradictory characteristics such that the member for connecting the high temperature section and the low temperature section has, on the one hand, high heat conductivity and, on the other hand, low heat conductivity from the perspective of high efficiency. However, in the conventional stirling engine structure it is impossible to satisfy such contradictory requirements simultaneously, thus either one of the requirements has to be sacrificed.
As a method for increasing the thermal efficiency of the stirling engine in view of such technological background, for example, there is proposed a method in which a level difference is applied in a center position of a U-shaped bent portion of each of two adjacent heater tubes of a plurality of U-shaped heater tubes which perform heat exchange between combustion gas and working gas of a combustor, whereby a space of even width between the U-shaped tubes is secured at all times without allowing the U-shaped tubes to interact with each other even if receiving thermal stress or external pressure, and the high-temperature combustion gas can be evenly allowed to contact with the U-shaped tubes to increase the heat exchange efficiency of the high temperature section (see the patent document 1). There is also proposed a method in which a compression space and an expansion space are connected to each other by a plurality of connecting tubes, a low temperature section, a regenerating portion, and a high temperature section are disposed sequentially in each of the connecting tubes, and, by freely changing specification of the regenerating portion and of the low temperature section in accordance with the distribution of the temperatures of the high temperature section, the engine power is improved (see the patent document 2). Furthermore, there is proposed another method in which a high temperature section, a regenerator, and a low temperature section are surrounded by a double shell, and an incompressible heat insulating material such as liquid chlorine is filled into the double shell, whereby operating temperature and pressure are increased, efficiency of the regenerator is improved, and the number of times that heat is transferred in a direction perpendicular to the direction of flow of working fluid is increased (see the patent document 3).                Patent document 1: Japanese Patent Application Laid-open No. H5-172003        Patent document 2: Japanese Patent Application Laid-open No. H6-280678        Patent document 3: Japanese Unexamined Patent Publication No. 2001-505638        